Aluminum alloy in mold for tire and tire mold

ABSTRACT

An aluminum alloy for a tire mold comprises Mg: 3.0-6.0 mass %, Si: 0.2-4.5 mass % and the balance being Al and inevitable impurities.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an aluminum alloy suitable as a startingmaterial of a tire mold used in the vulcanization building of tires, andmore particularly to a casting aluminum alloy and a tire mold comprisingsuch an aluminum alloy.

2. Description of the Related Art

A tire mold (hereinafter referred to as “mold” simply), particularly aso-called split mold consisting of plural mold segments is commonly madefrom an aluminum alloy from a viewpoint of the casting property. As atypical example of the casting aluminum alloy used in the tire mold areknown AC2B, AC4C and AC7A defined in JIS H5202 (1992), which isdisclosed in Non-Ferrous Metals of JIS HANDBOOK. These aluminum alloysare easy in the plastic deformation because an elastic limit strainσ_(e) is as small as 0.04% in AC2B system, 0.10% in AC4C system and0.12% in AC7A system.

In the mold made of the aluminum alloy, therefore, local deformation iseasily caused because the elastic limit strain σ_(e) is small ascompared with that of steel material as a typical example of a moldmaterial.

In the mold for the tire building, since the opening-closing of the moldis carried out every the vulcanization building, the local deformationof the mold is caused in the repeated opening-closing operations,particularly in the fitting of the mold segments. The local deformationresults in the deformation of the inner face form of the mold, whichbrings about the deterioration of RR in the tire or the poor fittingamong the mutual mold segments in the split mold and hencedisadvantageously produces a rubber-protruded tire.

Such a local deformation of the mold is usually generated in about oneyear starting from the use in the vulcanization building of the tires,so that it is obliged to conduct the repairing of the mold at a cycle ofone year.

SUMMARY OF THE INVENTION

It is, therefore, an object of the invention to propose a way forenhancing an elastic limit strain of a casting aluminum alloy in orderto suppress the local deformation of the mold made of the aluminumalloy.

The construction of the invention is as follows.

(1) An aluminum alloy for a tire mold comprising Mg: 3.0-6.0 mass %, Si:0.2-4.5 mass % and the balance being Al and inevitable impurities.

(2) An aluminum alloy for a tire mold according to the item (1), whichfurther contains at least one of Cu: not more than 0.10 mass %, Zn: notmore than 0.10 mass %, Fe: not more than 0.10 mass %, Mn: not more than0.10 mass %, Ni: not more than 0.05 mass %, Ti: 0.01-0.30 mass %, Sn:not more than 0.05 mass %, Cr: not more than 0.10 mass %, B: not morethan 0.10 mass %, Ag: not more than 0.10 mass % and Ca: not more than0.10 mass %.

(3) A tire mold for use in vulcanization building of a tire, whichcomprises an aluminum alloy as described in the item (1) or (2).

According to the invention, the design for enhancing the elastic limitstrain can be given to the casting aluminum alloy. Therefore, by usingsuch an aluminum alloy can be improved mechanical properties of a castmold made of the aluminum alloy, and particularly the local deformationis suppressed during the vulcanization building, and hence it ispossible to prolong the service life of the mold.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As to the composition of the aluminum alloy according to the invention,the reason of the limitation on the each component is described indetail below.

Mg: 3.0-6.0 mass %

Mg is a solid-solution strengthening element and is included forstrengthening the grain boundary with Al—Mg precipitates. Also, Mghaving an atomic radius larger than that of Al is inhabited in thecrystal grain to form Guinier-Preston zone (GP zone), and the elasticitycan be maintained by the GP zone. That is, the GP zone is a stackingfault due to aggregation of solid-solution elements. The solid-solutionstrengthening by the GP zone appearingly serves as a dispersionstrengthening to trap dislocation causing loss of elasticity, and hencethe elastic limit increases and the strength rises. For this end, it isrequired to include nor less than 3.0 mass % of Mg. On the other hand,when the content of Mg exceeds 6.0 mass %, the stable GP zone is notformed and the scattering of the properties inclusive of elastic limitbecomes large every the cast lot and further the castability isconsiderably deteriorated and the precision casting is difficult.

Si: 0.2-4.5 mass %

Si contributes to strengthen the grain boundary by an eutectic systemwith Al. Since Al—Si eutectic is easily peeled off from Al matrix, it isrequired that Al—Mg—Si based inclusion is produced at the solidificationstage of the casting and bonded to the Al—Si eutectic to prevent thecrystal slippage at the grain boundary. For this end, it is required toinclude not less than 0.2 mass % of Si. Particularly, in order toproduce the Al—Mg—Si based inclusion at the solidification stage of thecasting, the aforementioned inclusion of not less than 3.0 mass % of Mgis required in addition to not less than 0.2 mass % of Si. That is, theAl—Mg—Si based inclusion produces Al—Mg₂Si fine precipitates, and theAl—Mg₂Si also forms the GP zone to bring about the improvement of theelastic limit likewise the above case. On the other hand, when itexceeds 4.5 mass %, the Al—Mg₂Si is preferentially precipitated in thegrain boundary to easily cause embrittlement, so that Si is required tobe not more than 4.5 mass %.

The balance other than Mg and Si is Al and inevitable impurities. In thealuminum alloy according to the invention, one or more of Cu, Zn, Fe,Mn, Ni, Ti, Sn, Cr, B, Ag and Ca may be included in addition to theabove components, if necessary. Among them, Sn, B, Ca and Cr areexistent as an inevitable impurity. A preferable content range of eachof these components is as follows.

Cu: not more than 0.10 mass %

Cu is preferably added in an amount of not less than 0.05 mass % inorder to stably produce the Al—Mg based GP zone and Al—Mg₂Si based GPzone, but when the addition amount of Cu exceeds 0.10 mass %, the aboveGP zones are segregated to make the scattering of the properties large,so that the upper limit is 0.10 mass %.

Zn: not more than 0.10 mass %

Zn is is preferably added in an amount of not less than 0.05 mass % inorder to stably produce the Al—Mg based GP zone and Al—Mg₂Si based GPzone, but when the addition amount of Zn exceeds 0.10 mass %, there is afear that Al—Mg₂Si is preferentially precipitated in the grain boundaryto induce the intergranular cracking, so that the upper limit is 0.10mass %.

Fe: not more than 0.10 mass %

A slight amount of Fe added leads to the improvement of elastic limit,so that Fe is preferably added in an amount of not less than 0.05 mass%. However, when the addition amount exceeds 0.10 mass %, thecastability is obstructed, so that the upper limit is 0.10 mass %.

Mn: not more than 0.10 mass %

Mn is preferably added in an amount of not less than 0.05 mass % forimproving the heat resistance, but when the addition amount exceeds 0.10mass %, the castability is obstructed, so that the upper limit is 0.10mass %.

Ni: not more than 0.05 mass %

Ni is preferably added in an amount of not less than 0.03 mass % forimproving the heat resistance and elastic limit, but when the additionamount exceeds 0.05 mass %, the castability is obstructed, so that theupper limit is 0.05 mass %.

Ti: 0.01-0.30 mass %

Ti is added in an amount of not less than 0.01 mass % for forming finegrain boundary, but when the addition amount exceeds 0.30 mass %, theeffect of forming the fine grain boundary is unchangeable, so that theupper limit is 0.30 mass %.

Sn: not more than 0.05 mass %

Sn is incorporated as an inevitable impurity, but when the contentexceeds 0.05 mass %, the castability is obstructed, so that the contentis controlled to not more than 0.05 mass %.

Cr: not more than 0.10 mass %

Cr is also incorporated as an inevitable impurity, but when the contentexceeds 0.10 mass %, the properties are obstructed, so that the contentis controlled to not more than 0.10 mass %.

B: not more than 0.10 mass %

B is also incorporated as an inevitable impurity, but when the contentexceeds 0.10 mass %, the properties, particularly elongation areobstructed, so that the content is controlled to not more than 0.10 mass%.

Ag: not more than 0.10 mass %

Ag is preferably added in an amount of not less than 0.01 mass % formaking finer the size of the polycrystalline alloy up to nano-size, butwhen the content exceeds 0.10 mass %, the effect of making the sizefiner is saturated, so that the upper limit is 0.10 mass %.

Ca: not more than 0.10 mass %

Ca is incorporated as an inevitable impurity, but when the contentexceeds 0.10 mass %, the castability is obstructed, so that the contentis controlled to not more than 0.10 mass %.

The aluminum alloy having the aforementioned composition is firstproduced as an ingot, which is cast through a melting furnace and aholding furnace to form a cast slab and then the resulting cast slab isreshaped through elastic and plastic deformations and finally subjectedto a machine work to form a mold.

Thus, there can be obtained a mold having an elastic limit strain σ_(e)of not less than 0.20%. When the vulcanization building of the tire iscarried out by using the split mold formed by fitting the mold segments,an average fitting strain is 0.04-0.07%, but the end face is locallydeformed in the fitting of the mold segments to generate strain inputcorresponding to 2-3 times of the average fitting strain. Therefore, inorder to attain the fitting at an elastic zone, the material used forthe mold is required to have an elastic limit strain of 0.08-0.21%, andhence the elastic fitting is attained when the elastic limit strain asthe mold is not less than 0.20%.

EXAMPLES

Each of mold segments used for the vulcanization building of a passengercar tire having a tire size of 225/60 R15 is cast from an aluminum alloyhaving a chemical composition shown in Table 1 under conditions shown inTable 2. As regards the castability, the state of generating castcavities is examined under a proper casting design. In this case, thecasting design is a gravity casting with a chill type ring of one sprue.

Then, the elastic limit strain is measured with respect to the resultingmolds. Furthermore, the vulcanization building of tires is conductedthrough the above molds under the following conditions over a long timeof period to measure spue generating ratio and mold repairing cycleafter the use period of one year. The measured results are shown inTable 3.

Conditions of tire vulcanization building (gas vulcanization)

-   -   platen/jacket temperature: 180° C./180° C.    -   initial stage of vulcanization: steam, pressure 1.67 MPa (17        kgf/cm²), 2.5 minutes    -   middle stage of vulcanization: inert gas (N₂ gas), pressure 2.06        MPa (21 kgf/cm²), 12 minutes    -   last stage of vulcanization: evacuation of inert gas up to a        pressure of 0.34 MPa (3.5 kgf/cm²) for 1 minute

Moreover, the ratio of generating cast cavities under the proper castingdesign is evaluated by a man-hour for repairing cavities, in which acase that the man-hour required for repairing one set of cavities isless than 4 hours is a small ratio (◯), a case that the man-hour is 4-8hours is a middle ratio (Δ) and a case that the man-hour exceeds 8 hoursis a large ratio (×).

The elastic limit strain is a proportional limit strain determined froma stress-strain curve obtained by a tensile test of a JIS No. 4 testpiece.

The spue generating ratio is a ratio of molds generating spues among 10molds. In this case, the spue-generated mold is determined when spues ofnot less than 1 mm are generated in the fitted portion between moldsegments in the mold.

Also, the mold repairing cycle means a cycle required for repairing thespue-generated mold in the continuous use of the mold, which isevaluated from the mold-repairing result.

TABLE 1 (mass %) Alloy No. Mg Si Cu Zn Fe Mn Ni Ti Sn Cr B Ag Ca RemarksA 4.0 1.0 0.1 0.1 0.1 0.1 0.05 0.05 0.05 0.1 0.1 0.1 0.01 InventionExample B 4.0 0.1 0.1 0.1 0.1 0.1 0.05 0.05 0.05 0.1 0.1 0.01 0.01Comparative Example (corresponding to AC7A) C 3.0 0.2 0.1 0.1 0.1 0.10.05 0.05 0.05 0.1 0.1 0.1 0.01 Invention Example D 6.0 4.5 0.1 0.1 0.10.1 0.05 0.05 0.05 0.1 0.1 0.1 0.01 Invention Example E 3.0 4.5 0.1 0.10.1 0.1 0.05 0.05 0.05 0.1 0.1 0.1 0.01 Invention Example F 6.0 0.2 0.10.1 0.1 0.1 0.05 0.05 0.05 0.1 0.1 0.1 0.01 Invention Example G 4.0 3.00.1 0.1 0.1 0.1 0.05 0.05 0.05 0.1 0.1 0.1 0.1 Invention Example H 2.00.2 0.1 0.1 0.1 0.1 0.05 0.05 0.05 0.1 0.1 0.1 0.1 Comparative Example I6.0 5.0 0.1 0.1 0.1 0.1 0.05 0.05 0.05 0.1 0.1 0.1 0.1 ComparativeExample J 2.0 4.5 0.1 0.1 0.1 0.1 0.05 0.05 0.05 0.1 0.1 0.1 0.1Comparative Example K 7.0 0.2 0.1 0.1 0.1 0.1 0.05 0.05 0.05 0.1 0.1 0.10.1 Comparative Example L 7.0 4.5 0.1 0.1 0.1 0.1 0.05 0.05 0.05 0.1 0.10.1 0.1 Comparative Example M 7.0 0.1 0.1 0.1 0.1 0.1 0.05 0.05 0.05 0.10.1 0.1 0.1 Comparative Example N 7.0 5.0 0.1 0.1 0.1 0.1 0.05 0.05 0.050.1 0.1 0.1 0.1 Comparative Example

TABLE 2 Sam- Alloy Casting Casting Cover chill ple symbol temperaturerate temperature No. for mold (° C.) (kgf/min) (° C.) Remarks 1 A 720100 300 Invention Example 2 B 720 100 300 Comparative Example 3 C 720100 300 Invention Example 4 D 720 100 300 Invention Example 5 E 720 100300 Invention Example 6 F 720 100 300 Invention Example 7 G 720 100 300Invention Example 8 H 720 100 300 Comparative Example 9 I 720 100 300Comparative Example 10 J 720 100 300 Comparative Example 11 K 720 100300 Comparative Example 12 L 720 100 300 Comparative Example 13 M 720100 300 Comparative Example 14 N 720 100 300 Comparative Example

TABLE 3 Castability Evaluation Elastic Spue Mold Alloy for limit gen-repair- symbol generating strain erating ing Sample for cast σ_(e) ratiocycle No. mold cavities (%) (%) (years) Remarks 1 A ◯ 0.24 10 5Invention Example 2 B X 0.08 30 2 Comparative Example 3 C ◯ 0.25 12 5Invention Example 4 D ◯ 0.26 8 5 Invention Example 5 E ◯ 0.25 10 5Invention Example 6 F ◯ 0.28 5 5 Invention Example 7 G ◯ 0.27 5 5Invention Example 8 H X 0.06 40 1 Comparative Example 9 I X 0.25 12 5Comparative Example 10 J Δ 0.07 30 2 Comparative Example 11 K X 0.26 8 2Comparative Example 12 L Δ 0.28 5 5 Comparative Example 13 M X 0.09 251.5 Comparative Example 14 N X 0.27 5 5 Comparative Example

1. An aluminum alloy for a tire mold comprising Mg: 3.0-6.0 mass %, Si:0.2-4.5 mass % and the balance being Al and inevitable impurities.
 2. Analuminum alloy for a tire mold according to claim 1, which furthercontains at least one of Cu: not more than 0.10 mass %, Zn: not morethan 0.10 mass %, Fe: not more than 0.10 mass %, Mn: not more than 0.10mass %, Ni: not more than 0.05 mass %, Ti: 0.01-0.30 mass %, Sn: notmore than 0.05 mass %, Cr: not more than 0.10 mass %, B: not more than0.10 mass %, Ag: not more than 0.10 mass % and Ca: not more than 0.10mass %.
 3. A tire mold for use in vulcanization building of a tire,which comprises an aluminum alloy as claimed in claim
 1. 4. A tire moldfor use in vulcanization building of a tire, which comprises an aluminumalloy as claimed in claim 2.